phys10 chap6 work&kineticenergy

7/26/2019 Phys10 Chap6 Work&KineticEnergy

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WORK AND KINETIC ENERGY

-> any activity that requires muscular or ment al effort

-> done by exerting force on an object w/c then undergoes displacement

-> requires energy

dFW

Fdcos

-> SI unit

joule

(J) after James Prescott Joule

mN1J1

||FddFW ||

3

(positive)W

(negative)W

0W

Work is done

ON

the body

Work is done

BY

the body

Work is done

BY

the one

exertingforce

the body

Work is done

ON

the one

exertingthe body

Zero work done

d

d

d

Gravity

is really the one

doing the work!

d

d


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5

Example

1:

Work

done

by

a

constant

force

Steve

exerts

a

steady

force

of

210

N

on

the

stalled

car

as

he

pushes

it

a

distance

of

18

m.

The

caralso has a flat tire, so to make the car move

straight,

Steve

must

push

at

an

angle

of

30.

0

o

tothe

direction

of motion.

(a)

How

much

work

doesSteve

do?

(b)

In a helpful mood,

Steve

pushes

a

second

stalled

car

with

a

steady

force

of

The

displacement

of

the

car

is

How

much

work

does

Steve

do

in

this

case?

jiF )N40()N160(

jid )m11()m14(

F

Example

2:

Work

done

by

several forces.

A

farmer

hitches

his

tractor

to

a

sled l

oaded

withfirewood

and

pulls

it

a

distance

of

20

m

along l

evelground.

The

total weight of

the

sled

and l

oad

is14700

N.

The

tractor

exerts

a

5000-

N

force

at

anangle

of 36.

9

o above

the

horizontal.

There is a3500-

N

frictional force

opposing

the

motion.

Find

the

work

done

by

each

force

acting

on

the

sled

andthe

total work

done

by

all the

forces.

7

Example

3:

Work

done

by

gravity.

A

75

kg

painter

climbs

a l

adder

that

is

2.

75

m l

ongleaning

against

a

vertical wall.

The l

adder

makes

anangle

of

30.

0

o

with

the

wall. (a)

How

much

work

doesgravity do on the

painter?

(b)

does the

answer

inpart (a) depend

on whether

the

painter

climbs

atconstant

speed

or

accelerates

up

the l

adder? 30.0

o

->

energy

due

to

motion

(speed)

2m

2

1vK

Work-

Energy

Theorem

for

Kinetic

Energy

iftot KKKW


3/5

9

20

m

v

i v

f

Example

4:

Suppose

the

14700-

N

sled

has

an

initial velocity

of

2

m/s.

What

is

its

speed

after

itmoved

20

m?

(

W

tot

=

10

kJ

)

Example

5:

Comparing

kinetic

energies

Two

iceboats

hold

a

race

on

a

frictionless l

ake.

The

two

iceboats

have

masses

m

and

2

m.

Eachiceboat

has

an

identical sail,

so

the

wind

exertsthe

same

constant

force

on

each

of

them.

Thetwo ice

boats

start

from rest

and

cross

thefinish l

ine

a

distance

s away.

Which

ice

boatcrosses the finish line with greater

kineticenergy?

11

Example

6:

Finding

force

using

work-

energy

theorem

A

sled

with

mass

8.

00

kg

moves

in

a

straight l

ine

on

a

frictionless

horizontal surface.

Atone

point

in its

path,

its speed

is

4.

00

m/s

;

after

it

has

travelled

2.

50

m

beyond

thispoint,

its

speed

is

6.

00

m/s.

Find

the

constant

force

acting

on

the

sled.

2.50m

4.00 m/s 6.00 m/s

F F

Example

7:

Finding

the

initial velocity

using

work

energy

theorem

You

are

a

member

of

a

rescue

team

and

you

must

project

a

box

of

supplies

up

an

iof

constant

slope

angle

of

30.

0

o

so

that

it

reaches

a

stranded

skier

who

is

at

a

vedistance

of

2

m

above

the

bottom

of

the

incline.

What

is

the

minimum

speed

tha

must

give

the

box

so

that

it

will reach

the

skier

if

(a)

the

incline

is

frictionless?

(b

incline

has

a

coefficient

of

kinetic

friction

of

0.

20

?

30.0

o

v

i


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13

->

time

rate

of

doing

work

->

time

rate

of

expending

energy

t

WP

->

SI

unit

watt

(W)

average

power

s

J11W W7461hp

hp

=

horsepower

vFP

instantaneous

power

Example

8:

Power

climb

A

50

kg

marathon

runner

runs

up

the

stairs

to

thetop of Chicagos 443-

m tall SearsTower.

To l

ift

herself

to

the

top

in

15.

0

minutes,what must be her average

power inwatts?

In

kilowatts?

In

horsepower?

15

Example

9:

Force

and

power

Each

of the

two jet engines

in a Boeing 767 airliner

develops

a thrust

of 197

,

000

N(

44

,

300 l

b).

When

the

airplane

is

flying at

250

m/s,

what

horsepower

does

each

enginedevelop?

Prob

1.

A

factory

worker

pushes

a

30.

0

kg

crate

a

distance

of

4.

5

m

along

a

level floor

at velocity

by

pushing

horizontally

on

it.

The

coefficient

of

kinetic

friction

between

the

crate

floor

is

0.

25.

(a)

What

magnitude

of

work

must

the

worker

apply?

(b)

How

much

work

is

don

force?

(c)

How

much

is

the

work

done

by

friction?

By

gravity?

By

the

normal force?

(d)

Wh

total

work

done

on

the

crate?

Prob

2.

Two

blocks

are

connected

by

a

very li

ght

string

passing

over

a

massless

and frictionle

Travelling

at constant

speed,

the

20

-

N

block

moves

75

cm

to the right and the 12

-

N

blo

75

-

cm

downward.

During

this

process,

how

much

work

is

done (a)

on

the

12

-

N

block

by

(

(ii) the tension in

the

string? (b)

On the 20

-

N

block

by

(i

)

gravity,

(ii)

the tension in

t(iii)

friction,

and (iv)

the

normal force?

(c)

Find

the

total work

done

on

each

block.


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17

Prob3.Youthrowa20-Nrockvertically intotheairfromgroundlevel.Youobservethatwhenitis15mabovetheground,itistravelling25m/supward.Findthe rocksspeedjustasitleftthegroundand itsmaximumheight.

Prob4.Ablockof i cewithmass2kgslides0.750mdownan inclinedplanethatslopesdownwardatanangleof36.9o belowthehorizontal.Iftheblockoficestartsfromrest,whatis itsfinalspeed?

Prob5.Howmanyjoulesofenergydoesa100-watt light bulbuse perhour?Howfastwoulda70-kgpersonhavetoruntohavethatamountofkineticenergy?

Prob6.Anelevatorhasmass600kg,not includingthepassengers.Theelevatorisdesignedtoascend,atconstantspeed,avertical distanceof 20m(5floors)in16s,andit isdrivenbyamotorthatcanprovide upto40hpto the elevator. What is the maximumnumberof passengersthat canridetheelevator?Assumethatanaveragepassengerhasmass65kg.

phys10 chap6 work&kineticenergy

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